This invention relates to a nerve-stimulating needle with fingertip control of stimulator current output and visual as well as electrical depth determination ability.
Anesthesiologists commonly use nerve stimulators and insulated needles for the purpose of locating peripheral nerves, or nerve plexuses, for the performance of regional anesthesia procedures. This procedure is discussed in Vloka J D et al., xe2x80x9cA National Survey On Practice Patterns In The Use Of Peripheral Nerve Stimulators In Regional Anesthesia,xe2x80x9d The Internet Journal of Anesthesiology, Vol. 3, No. 4, 1999. In addition to targeting local anesthetic delivery for regional anesthesia, the use of nerve stimulators reduces the potential for nerve injury since direct contact with the nerve is not required for performance of the anesthetic. This is in contrast to the alternative method of seeking paresthesias to confirm needle position. Paresthesias are provoked by directly disturbing the nerve. If such needle to nerve contact can be avoided, direct needle trauma should be reduced.
The available nerve stimulators have differing output characteristics. The shape of the pulse is typically square or nearly so. The pulse widths vary from 40 microseconds (xcexcs) to 2 milliseconds (ms). Frequency selections for these stimulators range from 1.0 Hertz (Hz) to 100 Hz, in step gradations rather than continuously. Stimulators that are manufactured specifically for regional anesthesia applications typically do not have frequency options greater than 5 Hz. The stimulators that serve as both nerve stimulators and neuromuscular blockade monitors offer higher frequency choices, typically 50 and 100 Hz. These stimulators are discussed in Barthram CN, xe2x80x9cNerve Stimulators For Nerve Locationxe2x80x94Are They All The Same?,xe2x80x9d Anaesthesia, Vol. 52, 1997, pp. 761-764.
The needles used for stimulator assisted regional anesthesia typically have a molded plastic hub that contains both a connection to plastic tubing and a wire attached to the metal needle imbedded in the hub. This wire, when connected to a source, supplies electrical current to the needle with appropriate output characteristics for generation of action potentials in axons. Use of this equipment requires a means for holding the needle assembly, adjusting the output current, and injecting medication. This is awkward for a single user to accomplish and usually requires the presence of an assistant.
The commercially available nerve stimulators offer two methods of controlling the current output from the nerve stimulator instrument to a nerve stimulator needle. The first method is by hand-operated dials on the face of the nerve stimulator instruments. In this method, it is difficult for a single operator to insert and position the needle in the patient, control the current supplied the nerve stimulator needle, and inject the medication in the patient.
The second method is by a foot-operated pedal connected via a cable to the nerve stimulator instrument. An output source with foot-pedal control, such as described in U.S. Pat. No. 5,830,151 to Hadzic, necessitates a multiplicity of wires connecting pieces of equipment together. In the environment of an operating room where a multiplicity of electrical cables already exists, any equipment that adds additional cables spread out across the floor or tables and carts represents increased hazard for stumbling and consequent injury. Also, efficiency of movement is highly prized in the operating room environment. When an anesthetic procedure is completed, the operating room personnel move rapidly to begin the surgical positioning and prepping. Often, the anesthesiologist is in the position of gathering up equipment used for a procedure, and either disposing of it or placing it on a cart for subsequent storage. Tangles of cables and wires complicate this process and have a tendency to increase the clutter surrounding anesthesia machines and carts.
U.S. Pat. No. 4,515,168 to Chester et al. discloses to clamp a nerve stimulator and locating device onto a syringe. As the entire nerve stimulator device is clamped onto the syringe, the unit is a long and clumsy assembly, which is difficult to maneuver. Moreover, the device disclosed by Chester does not allow for one-handed operation of needle advancement and current control. Particularly, the needle is advanced by one hand while the current must be controlled by turning the knob 27 with the other hand, which is an extremely awkward operation for the user. Additionally, the nerve stimulator of the Chester patent restricts the size of the syringe upon which it may be mounted, thus, requiring the operator to change the syringe on the needle. This combination makes it very difficult to stabilize the needle within 1-2 mm of a nerve as desired for a regional block.
U.S. Pat. No. 5,306,236 to Blumenfeld et al. discloses a handle 36 to which the syringe, the needle and a conductor for carrying an electrical signal are attached. The control mechanism for controlling the application of current to the needle is located remotely from the needle at a stimulator device. Like the Chester patent, the system of Blumenfeld also does not allow for one-handed operation of both needle advancement and current control. Accordingly, the system of the Blumenfeld patent also provides a clumsy operation for the user.
It is frequently useful, during and after the performance of a regional anesthetic procedure, to know the depth at which the nerve structure was located. The consideration of needle tip depth is valuable both for medical record purposes as well as a check on needle position during the performance of a procedure. Accurate initial needle position may be obtained, but then undergo alteration by displacement during the injection portion of the procedure. Displacement may be due to inadvertent pressure applied by the operator, or the tendency of the injected fluid to force the needle back along its tissue track. The presence of visual guides on the needle itself, or a read out of needle tip depth on the nerve stimulator device, would provide feedback to the operator so as to prevent needle displacement. At present, there is no mechanism for providing such information with the currently available needles designed for use with nerve stimulators.
In accordance with the present invention, an output control needle comprising an insulated hypodermic needle having a means for fingertip control of the stimulator output current and a means of depth determination. The invention will provide a means for a single operator, the anesthetist, to accurately position a needle and perform a regional anesthetic technique without need of an assistant or excessive cabling. This further reduces the costs associated with an assistant""s time while maintaining operating room hazards at a minimum.
It is an object of the present invention to provide a nerve stimulator needle with fingertip control of output current, which avoids the need for an assistant or foot pedal to adjust the output source during the performance of a regional anesthetic procedure.
It is an object of the present invention to provide a nerve stimulator needle with fingertip control of output current allowing the second hand to perform medication injection.
It is an object of the present invention to provide a nerve stimulator needle with a means of electronically measuring the depth at which a nerve structure is encountered.
It is an object of the present invention to provide a nerve stimulator needle with a means of visually measuring the depth at which a nerve structure is encountered.
In the present invention a nerve stimulator needle apparatus may comprise: a needle capable of carrying an electric current; a variable control mechanism which is operable to variably control the amplitude of an application of electric current to the needle; a plurality of electrical connectors connected to the variable control mechanism and the needle which allows the variable control mechanism to remotely connect to a nerve stimulation device which is operable to provide a voltage to the variable control mechanism and to provide a current pulse to the needle having an amplitude which is controlled by the variable control mechanism; and a housing which holds the variable control mechanism, the plurality of electrical connectors and the needle.
The needle may be an insulated hypodermic needle, and the apparatus may further comprise: an injection tube operably connected to the needle to provide a fluid to said needle. The housing may include a fluid path, and the injection tube may be connected to one end of the fluid path and the needle may be connected to another other end of said fluid path.
The variable control mechanism of the needle apparatus may include a pressure sensitive switching mechanism which changes the amplitude of the current applied to the needle in relation to the amount of pressure applied to the pressure sensitive switching mechanism.
The housing of the needle apparatus may further include conductive traces that connect the variable control mechanism and the needle to said electrical connectors.
The needle unit of the needle apparatus may include an electrically resistive layer covering the needle, wherein the resistance of the resistive layer changes with the length of the resistive layer, and a conductor associated with the housing may provide a voltage to the electrically resistive layer. The needle unit may further include an insulating layer between the electrically resistive layer and the needle.
The needle unit may further includes depth measurement marks that indicate the insertion depth of the needle.
The variable control mechanism of the needle apparatus may comprise a variable optical switching device. The variable optical switching device may include a partially colored plate and a fiber optic cable which directs light from a light source to impinge on the partially colored plate, and direct reflected light from the partially colored plate to a sensor. The partially colored plate preferably alters color or intensity component of the light impinged on it from said fiber optic cable in response to an application of pressure.
The variable optical switching device may include a graduated reflective plate and a fiber optic cable that directs light from a light source to impinge on the graduated reflective plate, and direct reflected light from the graduated reflective plate to a sensor. The graduated reflective plate preferably alters color or intensity component of the light impinged on it from the fiber optic cable in response to an application of pressure.
The invention may also include a nerve stimulator apparatus comprising: a nerve stimulator device comprising: a voltage source; a controller; and a current source operable to produce an electrical current in response to said controller; and a needle unit remotely located from said nerve stimulator device and connected to said nerve stimulator device only by at least one electrical conductor, said needle unit comprising: a variable control mechanism which is operable to receive a voltage from said voltage source and to provide instructions to said controller to variably control the amplitude of said electrical current provided by said current source; a needle capable of carrying said electric current from said current source having the amplitude controlled by said variable control mechanism; and a housing which holds said variable control mechanism and said needle.
The variable control mechanism may include a pressure sensitive switching mechanism that changes the amplitude of the current applied to the needle in relation to the amount of pressure applied to the pressure sensitive switching mechanism. The current source is preferably operable to increase or decrease current in response to the operation of the variable control mechanism, and the variable control mechanism is operable to increase or decrease current to the needle. The current source preferably changes the amplitude of the current in response to the operation of the variable control mechanism and the variable control mechanism is operable to control the rate of change of the current.
The needle unit may include an electrically resistive layer covering the needle, wherein the resistance of the resistive layer changes with the length of the resistive layer, and an electrical trace from an electrical connector provides a voltage from the voltage source, so as to maintain a constant current, to the electrically resistive layer. The microprocessor preferably determines the insertion depth of the needle according to the equation:
Lb=(Rtxe2x88x92Ra)/rL
where Ra is the resistance of the portion of the needle protruding above a skin surface of a subject; Rt is the resistance of the total length of the needle; rL is the resistance per unit length of the needle; and Lb is the insertion depth of the needle. Wherein the value Ra is calculated from the ratio of the voltage of a signal detected by a return electrode attached to the surface of the skin of a subject divided by the applied current of the resistive layer on the needle.
The invention further comprises a method of locating nerves may comprise the steps of: (A) inserting a needle mounted on a housing through a surface of skin of a subject; (B) activating a variable control mechanism mounted on the housing to provide a variably controlled current to the needle; (C) monitoring a detected current signal from a return electrode attached to the surface of skin of a subject using a nerve stimulator device,
wherein steps A and B are performed by the same hand of an operator.
The method of locating nerves may further comprise the step of determining the needle insertion depth by providing a voltage to a resistive layer, completing a circuit through a return electrode on the skin surface.
The step of determining the needle insertion depth is preferably performed according to the equation:
Lb=(Rtxe2x88x92Ra)/rL
where Ra is the resistance of the portion of the needle protruding above a skin surface of a subject; Rt is the resistance of the total length of the needle; rL is the resistance per unit length of the needle; and Lb is the insertion depth of the needle.
The determination of the needle depth may comprise the steps of: continuously applying a constant current to a resistive layer on the surface of the needle; periodically applying a current pulse to the needle while applying the constant current; waiting for a period of time to elapse after applying the current pulse to the needle and determining the resistance of the resistive layer of the needle exposed from the surface of skin of the subject; and calculating the depth of the needle inserted in the surface of skin of the subject.